Inverters are often used to convert DC power into AC power for one or more AC loads. For example, in a variable-speed, constant-frequency (VSCF) power conversion system, a brushless, synchronous generator converts variable-speed motive power produced by a prime mover into variable-frequency AC power. A rectifier converts the variable-frequency AC power into DC power and an inverter converts the DC power into constant-frequency AC power. The inverter may be of the stepped waveform type that produces three-phase output waveforms each comprising a series of steps or levels. Typically, each phase output waveform is subdivided into six or a multiple of six conduction periods in each cycle of the output, wherein the waveform assumes one of a plurality of constant levels in each conduction period.
Stepped waveform inverters produce output waveforms having harmonics that are more easily filtered than the harmonics produced in a pulse width modulated (PWM) waveform. Thus, the size and weight of a filter coupled to the inverter output may be reduced as compared to a PWM type of inverter.
Often, it is necessary to momentarily or continuously connect the outputs of inverters in parallel to a common load. For example, in an aircraft having multiple VSCF systems connected to a common load bus, it may prove necessary or desirable to transfer the load bus (and the loads connected thereto) from one VSCF system to another. In such a case, a no-break power transfer sequence is effected so that power flow to the loads is not interrupted. Alternatively, it may prove necessary or desirable to connect the VSCF systems in parallel in a continuous fashion so that both supply power to the loads. Recker, et al. U.S. Pat. Nos. 4,937,462 and 4,905,134, assigned to the assignee of the present application, disclose a no-break power transfer control and a paralleling control, respectively, for a PWM inverter in a VSCF system.
Szepesi, U.S. Pat. No. 4,535,399 discloses the use of a phase-locked loop (PLL) for controlling the output of an inverter that supplies AC power to a resonant load.
White, U.S. Pat. No. 4,779,184, assigned to the assignee of the present application, discloses a switched mode power supply having first and second sections wherein a PLL maintains the output of the second section at a precise 180.degree. relationship with respect to the output of the first section.
Compoly, U.S. Pat. No. 3,832,622 discloses phase-locked circuitry for operating a plurality of static inverters in synchronism.
Miyazawa, U.S. Pat. No. 4,733,341 discloses an apparatus for controlling first and second inverters such that the inverters produce outputs that are synchronized to a commercial power supply. Digital phase-locked loop oscillators compare the phase of the inverter outputs with the phase of the commercial power supply and dividers divide the outputs of the digital phase-locked loop oscillators by a variable factor N. The outputs of the dividers are coupled to ring counters that in turn control the inverters.
None of the foregoing patents discloses a control for a stepped waveform inverter, and more particularly a control which regulates the output frequency thereof so that the inverter may be connected momentarily or continuously in parallel with an AC power source.